What Is a Fistula in the Brain? Causes and Symptoms

A fistula in the brain is an abnormal direct connection between an artery and a vein, bypassing the tiny capillary network that normally slows and regulates blood flow. The most common type is called a dural arteriovenous fistula, or DAVF, which forms in the tough membrane (the dura mater) surrounding the brain. These lesions are uncommon, estimated to affect roughly 0.15 to 0.29 percent of the general population, but they can produce a surprisingly wide range of symptoms, from a rhythmic whooshing sound in one ear to seizures, vision problems, or even a form of reversible dementia.

How a Brain Fistula Differs From Other Vascular Problems

The brain’s blood supply normally flows from arteries into progressively smaller vessels until it reaches capillaries, where oxygen and nutrients exchange with brain tissue. Blood then collects into veins and drains away. A dural arteriovenous fistula short-circuits this system: arterial blood rushes directly into a venous channel, usually one of the large drainage sinuses within the dura. Because arteries carry blood at much higher pressure than veins are built to handle, this abnormal routing can engorge and distort the veins, raise venous pressure, and eventually compromise normal blood drainage from the brain itself.

DAVFs are distinct from arteriovenous malformations (AVMs), which are tangles of abnormal vessels embedded within the brain tissue. A DAVF sits in the dura, outside the brain proper, and is most often an acquired condition rather than something a person is born with. That said, the boundary between the two categories can blur in certain genetic conditions, as discussed below.

What Causes a Brain Fistula to Form

The precise chain of events that creates a DAVF is still not fully understood, but the dominant theory centers on venous sinus thrombosis, a blood clot in one of the brain’s major drainage channels. When a clot blocks a sinus, the surrounding tissue experiences a spike in venous pressure. The body responds by growing new small blood vessels in the dura, a process driven by signaling molecules such as vascular endothelial growth factor (VEGF). Animal research has shown that artificially raising VEGF levels in rats with venous hypertension increases new blood vessel growth in the dura, while blocking the VEGF receptor reduces the rate at which fistulas develop.¹ Other angiogenic growth factors released during the healing process after sinus thrombosis may also play a role.²

Anything that predisposes someone to venous sinus thrombosis can therefore raise the risk of eventually developing a DAVF. Head trauma, prior brain surgery, ear infections, hypercoagulable states, and pregnancy have all been implicated. In a clinical trial studying patients who had already experienced cerebral venous thrombosis, researchers looked specifically for new fistula formation on follow-up imaging and found it to be rare in that cohort, with only one case already present at baseline.³ That finding suggests that while venous thrombosis is a known trigger, the progression from clot to fistula does not happen in most people.

Genetic and Congenital Links

A small proportion of brain fistulas appear in people with hereditary hemorrhagic telangiectasia (HHT), a genetic condition that causes fragile, malformed blood vessels throughout the body. A systematic review found the prevalence of dural fistulas in HHT patients to be about 1.2 percent, substantially higher than the background rate in the general population.⁴ In a separate study of 321 HHT patients, about 3.7 percent had a cerebrovascular malformation of some kind, with one patient specifically diagnosed with a dural arteriovenous fistula.⁵ These numbers are small, but the excess over the general-population rate supports the idea that shared genetic pathways can predispose certain people to fistula development.

Common Symptoms and How They Vary by Location

The symptoms a brain fistula produces depend heavily on where it sits and, critically, on how the blood drains out of it. A fistula near the back of the skull that drains into the transverse or sigmoid sinus will produce very different complaints than one near the eye that drains into the cavernous sinus. Some fistulas cause no symptoms at all and are discovered incidentally during imaging for something else. Others can be life-threatening. The pattern of venous drainage seen on vascular imaging correlates with both symptom severity and the risk of hemorrhage.⁶

Pulsatile Tinnitus

One of the most recognizable symptoms is pulsatile tinnitus: a rhythmic whooshing or pounding sound in one ear that matches your heartbeat. This happens because high-pressure arterial blood rushing through the fistula near the inner ear creates an audible turbulence. It is especially common when the fistula involves the transverse or sigmoid sinus, which sit close to the ear structures. In one case series, more than 10 percent of patients with dural fistulas presented with pulsatile tinnitus as their only symptom.⁷ A 69-year-old man described in a recent case report presented with left-sided pulsatile tinnitus as his sole complaint; imaging revealed a transverse-sigmoid sinus fistula with complex internal anatomy.⁸ Because pulsatile tinnitus has many benign causes, a fistula may not be suspected immediately, especially when the patient first sees an ear specialist rather than a neurologist.

Eye Symptoms

When a fistula involves the cavernous sinus, which wraps around the area just behind each eye, it can cause dramatic ophthalmological symptoms. Elevated venous pressure in this area congests the veins draining the eye, leading to a red, swollen, bulging eye (proptosis), swelling of the membrane over the white of the eye (chemosis), and double vision from cranial nerve palsies affecting eye movement.⁹ These symptoms can come on gradually and may initially be mistaken for thyroid eye disease, orbital tumors, or severe conjunctivitis. A key distinguishing feature is that the eye findings typically affect one side and may be accompanied by an audible bruit, a sound that can sometimes be heard with a stethoscope placed over the eye or temple.

Seizures, Stroke-Like Episodes, and Hemorrhage

Higher-grade fistulas, those that redirect blood backward into cortical veins rather than forward into the large sinuses, carry a much more serious risk profile. This retrograde flow, called cortical venous reflux, raises pressure inside veins that were never designed for it. The consequences can include seizures, focal neurological deficits that mimic a stroke, and intracranial hemorrhage. One case described a patient whose new-onset seizures and post-seizure weakness led to imaging that revealed hemorrhages in the brain consistent with venous congestion from a dural fistula.¹⁰

A study tracking patients whose fistulas had persistent cortical venous reflux found the yearly mortality rate was about 10 percent. Outside of events present at the time of diagnosis, the annual risk of bleeding or non-hemorrhagic neurological problems during follow-up was roughly 15 percent combined.¹¹ Those numbers underscore why the drainage pattern matters so much: a fistula that drains benignly into a large sinus may be a nuisance, while one that forces blood into cortical veins is a medical emergency.

The Surprising Link to Reversible Dementia

One of the more overlooked presentations of a brain fistula is progressive cognitive decline that looks, on the surface, like Alzheimer’s disease or another neurodegenerative condition. When venous hypertension from a DAVF backs up into the deep medullary veins, it can cause widespread swelling in the brain’s white matter. This shows up on MRI as bilateral white matter changes, a pattern easily mistaken for vascular dementia, multiple sclerosis, or other conditions.¹²

The critical point is that this form of dementia is reversible once the fistula is treated. Case series have documented patients whose memory, orientation, and executive function returned substantially after the abnormal connection was closed off.¹³ This remains a rare presentation, and that very rarity means clinicians sometimes do not think to look for it. The practical takeaway: if someone develops progressive cognitive decline with bilateral white matter changes on imaging but the clinical picture does not quite fit typical dementia, a dural fistula should be on the differential diagnosis.

How Brain Fistulas Are Diagnosed

The gold standard for confirming a dural arteriovenous fistula is catheter-based digital subtraction angiography (DSA), a procedure in which a thin catheter is threaded through a blood vessel and contrast dye is injected to produce detailed, real-time images of blood flow.¹⁴ DSA gives the clearest picture of where the fistula is, which arteries feed it, and how the blood drains out, all of which are essential for deciding on treatment. Advances in noninvasive imaging, including CT angiography and MRI with specialized techniques like arterial spin labeling, have made it possible to detect or suspect a fistula without an invasive procedure.¹⁵ In practice, CT or MRI often serves as the first step, with DSA reserved for confirmation and pre-treatment planning.

Treatment Options

The management of brain fistulas spans a wide range, from simple monitoring to catheter-based embolization, open surgery, and focused radiation. Which approach is appropriate depends on the fistula’s grade, its location, its drainage pattern, and the patient’s overall health. Multiple classification systems exist to stratify risk, with the common thread being that fistulas draining into cortical veins carry higher danger and are treated more aggressively than those draining into sinuses.¹⁶

Endovascular Embolization

The most widely used treatment today is endovascular embolization, in which a catheter is guided through the blood vessels to the fistula site and a liquid embolic agent is injected to seal the abnormal connection. The material most commonly used in recent decades is a substance called Onyx, though newer agents like Squid and PHIL have been developed to address some of Onyx’s limitations, including imaging artifacts and loss of visibility during long injections.¹⁷

Cure rates with Onyx embolization are high. A large review examining outcomes over two decades found that complete obliteration was achieved in roughly 88 percent of patients on immediate post-procedure imaging. When performed through an arterial approach alone, the complete obliteration rate was about 90 percent; a venous approach achieved roughly 96 percent closure.¹⁸ Earlier and smaller series had reported cure rates in a similar range, around 84 percent in one study of 25 patients.¹⁹ These numbers make embolization the first-line treatment for most DAVFs.

Surgery and Radiosurgery

Open surgery, once the mainstay, has become less common as endovascular techniques have improved. It is still used in cases where embolization is not feasible or has failed. Gamma Knife radiosurgery, a form of highly focused radiation, offers a noninvasive alternative and has shown effectiveness in symptom control, though it works more slowly than embolization. One center reported that the median time from radiosurgery to symptom relief was about 14 months, and the median time to confirmed obliteration on imaging was about 45 months.²⁰ For higher-grade fistulas that carry an immediate hemorrhage risk, a combined strategy of embolization followed by radiosurgery may offer the best of both worlds: quick risk reduction plus long-term closure.

When Watching and Waiting Makes Sense

Not every brain fistula needs to be treated right away. Low-grade fistulas, those that drain into a venous sinus without cortical venous reflux, are generally considered benign. A pooled analysis of 469 patients compared intervention to observation for this type of fistula and found no significant difference in fistula grade progression, symptom progression, or functional independence at follow-up. Perhaps more strikingly, the intervention group actually had a higher rate of serious adverse events: about 4 percent experienced a fistula-related death or permanent complication from treatment, compared with zero percent in the observation group.²¹ A separate analysis reached a consistent conclusion: intervention for low-grade fistulas achieves better obliteration on imaging but does not improve neurological or functional outcomes.²²

This does not mean low-grade fistulas should be ignored. Periodic imaging follow-up is important because a small percentage can progress to a higher grade over time. But the evidence suggests that if your fistula is low-grade and you have tolerable symptoms or none at all, the risks of treatment may outweigh the benefits. The decision is different for higher-grade fistulas with cortical venous reflux, where the annual risks of hemorrhage and neurological decline make treatment clearly worthwhile.

Fistulas in Newborns and Children

A distinct and particularly dangerous type of brain fistula can occur in newborns: the vein of Galen aneurysmal malformation (VGAM). This is a large fistula involving a deep venous structure in the center of the brain, and it is congenital, meaning it develops before birth. Because the fetus shares blood flow with the placenta, the low-resistance fistula may not cause serious problems in utero. After birth, when the placental circulation is cut off and the baby’s systemic blood pressure rises, a massive volume of blood gets diverted through the fistula. This steals blood from the rest of the body and overwhelms the right side of the heart.²³

Newborns with large VGAMs can develop severe heart failure within hours or days of birth. A characteristic finding on ultrasound is reversed blood flow in the descending aorta during diastole, meaning blood is being sucked backward toward the fistula instead of flowing forward to the body. Studies of neonates with severe VGAM-related heart failure have found that cardiogenic shock, signs of right ventricular failure, and persistent open ductus arteriosus were significantly more common in babies who did not survive.²⁴ Treatment involves staged embolization, and in survivors, the ratio of pulmonary to systemic arterial pressure dropped significantly after the first embolization procedure. These cases represent the extreme end of the brain fistula spectrum and require management in specialized pediatric neurovascular centers.

Why Brain Fistulas Are Often Misdiagnosed

A recurring theme in the medical literature on DAVFs is diagnostic delay. The symptoms are protean, meaning they mimic many other conditions. Pulsatile tinnitus gets attributed to ear problems. A red, bulging eye prompts a workup for thyroid disease. Progressive confusion triggers a dementia evaluation. Seizures lead to epilepsy testing. In each case, the fistula may not be considered until the more common diagnoses have been ruled out, and sometimes not even then. The dementia presentation is particularly treacherous because brain MRI may show white matter changes that look like a typical vascular or degenerative process, and the key to recognizing a fistula is looking for dilated cortical veins or abnormal vascular flow that a radiologist might not flag unless specifically looking for it.

The people most likely to get a timely diagnosis are those whose symptoms prompt vascular imaging early. Someone with new pulsatile tinnitus who gets a CT angiogram has a reasonable chance of being picked up. Someone whose first symptom is gradual cognitive slowing may wait months or years before anyone considers a vascular cause. If you or someone you know is experiencing unexplained neurological symptoms that do not fit a neat diagnostic box, asking about the possibility of a vascular malformation is reasonable, especially if symptoms are progressive and imaging shows bilateral white matter changes or unexplained venous congestion.